1. Field of the Invention
This invention is directed to acousto-optic devices, in general, and to acousto-optic spectrum analyzers which exhibit an extended dynamic range, in particular.
2. Prior Art
Acousto-optic (AO) spectrum analyzers are known in the art. A typical analyzer is shown and described in, inter alia, the copending application entitled ACOUSTO-OPTIC R-F FILTER WHICH IS TUNABLE AND HAS ADJUSTABLE BANDWIDTH, bearing U.S. Ser. No. 06/566,437, now abandoned by J. H. Labrum, filed on Dec. 28, 1983 and assigned to the common assignee. This apparatus and operation is well known in the prior art wherein a detailed description thereof is deemed unnecessary. This type of analyzer has many advantages over conventional spectrum analysis techniques. The major drawback of the AO spectrum analyzers which are currently known is that dynamic range of operation is limited to around 30-35 dB. Typically, the lower end of the dynamic range is determined by the "dark" current caused by reverse leakage through the photodetector diodes, while the high end is limited by photodetector saturation.
This is a severe limitation in the standard acousto-optic spectrum analyzers known in the art. It is desirable to have a photo-detector array with an extended dynamic range processing capability of the acousto-optic spectrum analyzer which can handle signals that have a dynamic range on the order of 60 or 70 dB. That is, the dark current limit is substantially fixed and not subject to variation. Thus, the upper range limit must be addressed in order to improve operation. However, with the existing components, it is impossible to achieve the improved operation utilizing of the known architecture in the standard acousto-optic processor.
The dynamic range of the photo-detector array is, by far, the single weak link in solving the instantaneous dynamic range problem. There are presently two definitions of "dynamic range" in common use and one must know which definition is being used before placing any significance on the numbers. The first definition is: EQU DR=20 Log (Vmax/Vmin) (EQ-1)
where:
Vmax is the detector output saturation voltage and PA1 Vmin is the detector output noise voltage. PA1 Vmax is the detector output saturation voltage and PA1 Vmin is the detector output noise voltage.
The second definition is: EQU DR=10 Log (Vmax/Vmin) (EQ-2)
Where:
Both equations use the same data but (EQ-1) yields twice the dynamic range of (EQ-2). In electronics, by definition, the first (EQ-1) equation is correct. However, because of the unique parameter being measured (optical power, not electrical power), the use of this equation yields the wrong result. In most applications, the photo-detector is an optical power-to-voltage converter. Thus, changing the optical input power by a factor of 10 results in an output voltage change of 10 times. Using equation (EQ-1) indicates a change of 2O dB when in fact the input optical power has only changed by 10 dB. The problem is really one of semantics, i.e., should one be measuring changes in the photo-detector output voltage or the optical power input to the detector? All references to the dynamic range of the detector in this description will be based on the use of (EQ-2). It is important to be aware of which equation is used in other publications before making any direct number comparisons.